From Wikipedia, the free encyclopedia

A monsoon is traditionally defined as a seasonal reversing wind accompanied by seasonal changes in precipitation,[1] but is now used to describe seasonal changes in atmospheric circulation and precipitation.[2] The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal may be debated.

The term was first used in English in British India (now India, Bangladesh and Pakistan) and neighbouring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area.[3] In hydrology, monsoon rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season. This allows other regions of the world to qualify as monsoon regions.[4]

Etymology and definition

The English monsoon came from Portuguese monção, ultimately from Arabicmawsim (موسم "season"), "perhaps partly via early modern Dutch monsun".[5] The Arabic-origin word mausam (मौसम, موسم) is also the word for "weather" in Hindi, Urdu, and several other North Indian languages.[6] The definition includes major wind systems that change direction seasonally.

Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapor in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside and cause drought.[7]

History

Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian sub-continent and Asia around 50 million years ago.[8] Many geologists believe the monsoon first became strong around 8 million years ago based on records from the Arabian Sea and the record of wind-blown dust in the Loess Plateau of China. More recently, plant fossils in China and new long-duration sediment records from the South China Sea led to a timing of the monsoon starting 15-20 million years ago and linked to early Tibetan uplift.[9] Testing of this hypothesis awaits deep ocean sampling by the Integrated Ocean Drilling Program.[10] The monsoon has varied significantly in strength since this time, largely linked to global climate change, especially the cycle of the Pleistocene ice ages.[11] Timing of the monsoon strengthening of the Indian Monsoon of around 5 million years ago was suggested due to an interval of closing of the Indonesian Seaway to cold thermocline waters passage from the Pacific to the Indian Ocean which is believed to have resulted in an increased sea surface temperature in the Indian Ocean, which increased gyral circulation and then caused an increased intensity of the monsoon.[12] Sinha et al. (2006) identified five episodes during the Quaternary at 2.22 Ma (PL-1), 1.83 Ma (PL-2), 0.68 Ma (PL-3), 0.45 Ma (PL-4) and 0.04 Ma (PL-5), of weakening of Leeuwin Current (LC) and postulated that the weakening of the LC would have an effect on the Sea Surface Temperature (SST) in the Indian Ocean, as the Indonesian throughflow generally warms the Indian Ocean. Thus these five intervals could probably be those of considerable lowering of SST in the Indian Ocean and would definitely have influenced Indian monsoon intensity. They (Sinha et al., 2006) stated that that during the weak LC there is the possibility of reduced intensity of Indian winter monsoon and strong summer monsoon, because of change in the Indian Ocean dipole due to reduction in net heat input to the Indian Ocean through the Indonesian throughflow. Thus a better understanding of the possible links between El Niño, Western Pacific Warm Pool, Indonesian Throughflow, wind pattern off western Australia, and ice volume expansion and contraction can be obtained by studying the behaviour of the LC during Quaternary at close stratigraphic intervals.[13]

Process

Monsoons may be considered as large-scale Sea breezes, due to seasonal heating and the resulting development of a thermal low over a continental landmass. They are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty metres deep, through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a metre or so. Additionally, the specific heat capacity of liquid water is significantly higher than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the air over the land warms faster and reaches a higher temperature than the air over the ocean. The hot air over the land tends to rise, creating an area of low pressure. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it.[14] Similar rainfall is caused by the moist ocean air being lifted upwards by mountains,[15] surface heating,[16] convergence at the surface,[17] divergence aloft, or from storm-produced outflows at the surface.[18] However as the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation.

In winter, the land cools off quickly, but the ocean retains heat longer. The cold air over the land creates a high pressure area which produces a breeze from land to ocean.[14] Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines, but they are much larger in scale, stronger and seasonal.[19]

As monsoons have become better understood, the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the Earth.

Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of supercontinents such as Pangaea, with their extreme continental climates.[citation needed]

Global monsoons

Africa

The monsoon of western Sub-Saharan Africa has traditionally been thought to be the result of the seasonal shifts of the Intertropical Convergence Zone and the great seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean.[20] It migrates northward from the equatorial Atlantic in February, reaches western Africa on June 22, then moves back to the south by October.[21] The dry, northeasterly trade winds, and their more extreme form, the harmattan, are interrupted by the northern shift in the ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid Sahel and Sudan depend upon this pattern for most of their precipitationed area is desert.

North America

The North American Monsoon (also abbreviated as NAM) occurs from late June or early July into September, originating over Mexico and spreading into the southwest United States by mid-July. It affects Mexico along the Sierra Madre Occidental as well as Arizona, New Mexico, Nevada, Utah, Colorado, West Texas and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of Southern California, but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast during the monsoon). The North American monsoon is known to many as the Summer, Southwest, Mexican or Arizona monsoon.[22][23] It is also sometimes called the Desert Monsoon as a large part of the affected area are the Mojave and Sonoran deserts.

Asia

The Asian monsoons may be classified into a few sub-systems, such as the South Asian Monsoon which affects the Indian subcontinent and surrounding regions, and the East Asian Monsoon which affects southern China, Korea and parts of Japan.

South Asian Monsoon

Southwest Monsoon

Onset dates and prevailing wind currents of the southwest summer monsoons in India.

The southwestern summer monsoons occur from June through September. The Thar Desert and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers, which causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas, creating winds blowing storm clouds towards the subcontinent. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, thus forcing them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain.

The southwest monsoon is generally expected to begin around the start of June and fade down by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topology, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.

The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India, thus making the area is the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats.

The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Eastern Himalayas in Shillong, India, is one of the wettest places on Earth. After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state[citation needed], pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala.

The monsoon accounts for 80% of the rainfall in India[citation needed]. Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s.

The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from the climax of summer heat in June.[24] However, the condition of the roads take a battering each year. Often houses and streets are waterlogged and the slums are flooded in spite of having a drainage system. This lack of city infrastructure coupled with changing climate patterns causes severe economical loss including damage to property and loss of lives, as evidenced in the Bombay floods of 2005. Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season. And in the recent past, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season.

The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang. It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i.e., monsoon-driven) origin (as opposed to "local convection").[25]

Northeast Monsoon

Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India, the Indian Ocean and its surrounding atmosphere still holds its heat. This causes the cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.

While travelling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Madras, which get less rain from the Southwest Monsoon, receives rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon.[26] In Southern Asia, the northeastern monsoons take place from December to early March when the surface high-pressure system is strongest.[27] The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system develops over South-East Asia and Australasia and winds are directed toward Australia known as a monsoon trough.

East Asian Monsoon

The East Asian monsoon affects large parts of Indo-China, Philippines, China, Korea and Japan. It is characterised by a warm, rainy summer monsoon and a cold, dry winter monsoon. The rain occurs in a concentrated belt that stretches east-west except in East China where it is tilted east-northeast over Korea and Japan. The seasonal rain is known as Meiyu in China, Changma in Korea, and Bai-u in Japan, with the latter two resembling frontal rain.

The onset of the summer monsoon is marked by a period of premonsoonal rain over South China and Taiwan in early May. From May through August, the summer monsoon shifts through a series of dry and rainy phases as the rain belt moves northward, beginning over Indochina and the South China Sea (May), to the Yangtze River Basin and Japan (June) and finally to North China and Korea (July). When the monsoon ends in August, the rain belt moves back to South China.

Australia

Also known as the Indo-Australian Monsoon. The rainy season occurs from September to February and it is a major source of energy for the Hadley circulation during boreal winter. The Maritime Continent Monsoon and the Australian Monsoon may be considered to be the same system, the Indo-Australian Monsoon.

The onset of the monsoon over the Maritime Continent tends to follow the heating maxima down Vietnam and the Malay Peninsula (September), to Sumatra, Borneo and the Philippines (October), to Java, Sulawesi (November), Irian Jaya and Northern Australia (December, January). However, the monsoon is not a simple response to heating but a more complex interaction topography, wind and sea, as demonstrated by its abrupt rather than gradual withdrawal from the region. The Australian monsoon or rainy season occurs in the austral summer when the monsoon trough develops over Northern Australia. Over three-quarters of annual rainfall in Northern Australia fall during this time.

Europe

The European Monsoon (more commonly known as the Return of the Westerlies) is the result of a resurgence of westerly winds from the Atlantic, where they become loaded with wind and rain.[28] These Westerly winds are a common phenomenon during the European winter, but they ease as Spring approaches in late March and through April and May. The winds pick up again in June, which is why this phenomenon is also referred to as "the return of the westerlies".[29]

The rain usually arrives in two waves, at the beginning of June and again in mid to late June. The European monsoon is not a monsoon in the traditional sense in that it doesn't meet all the requirements to be classified as such. Instead the Return of the Westerlies is more regarded as a conveyor belt that delivers a series of low pressure centres to Western Europe where they create unseasonable weather. These storms generally feature significantly lower than average temperatures, fierce rain or hail, thunder and strong winds.[30]

^ Official Web Site of District Sirsa, India. District Sirsa. Retrieved on 2008-12-27.

^Blumer, Felix P. (1998), [iahs.info/redbooks/a193/iahs_193_0343.pdf "Investigations of the precipitation conditions in the central part of the Tianshan mountains"], in Kovar, Karel, Hydrology, water resources and ecology in headwaters. Volume 248 of IAHS publication, International Association of Hydrological Sciences, pp. 343-350, ISBN 1901502457, iahs.info/redbooks/a193/iahs_193_0343.pdf

From LoveToKnow 1911

MONSOON (Arabic Mausim, season), the
name given to seasonal winds due to differences of pressure between
areas of land and sea, which are primarily caused by seasonal
differences of temperature. Monsoons may be regarded as the
seasonal analogue of the diurnal land and sea breezes. The term is,
however, also applied to seasonal winds which change in direction
on account of the migrations of wind-belts in the planetary
circulation. During the season of rising temperature the surface of
the land warms more quickly, and becomes hotter than that of the
sea, and during the season of falling temperature the reverse is
the case. Barometric pressure tends to be higher over the colder
region than over the warmer, and there is accordingly a tendency
for air to flow, in the lower levels
of the atmosphere,
from the former towards the latter. Thus there is in general a
movement from land to sea during the cold season, and from sea to
land during the warm season.

Within a belt extending from 10
to 15 degrees on each side of the equator, seasonal changes of temperature are
insufficient in range to permit of the occurrence of temperature
differences adequate to the development of true monsoons. In the
higher latitudes of the west wind-belt, and in the polar zones, the
generally low temperature does not favour the occurrence of wide
differences between land and sea. Thus the conditions required for
the occurrence of monsoonal winds are best satisfied in
intermediate latitudes in the neighbourhood of the tropics. But, as
in the case of land and sea breezes, the strength and extension of
the monsoon produced by the action described depends to a large
extent on the configuration of the land surface. When the land area
consists of a low plain, or of a plateau having a steep coastal strip of small width, the
circulation upon it tends to be local, and to approximate to the
typical "continental" climate of the temperate zones. Where, on the
other hand, the land slopes upwards gradually to a central massif or ridge the effect
of the differences of temperature is, as it were, cumulative, and
the monsoons may extend over large areas, affecting regions distant
from those in which the causes producing them are directly
operative, and the monsoon winds may develop great strength. Ferrel
(Popular Treatise on the Winds) has compared the
conditions in the two cases to those of a stove with a long horizontal flue and with a
vertical or inclined flue of the same length.

It is of course to be noted that the hot season monsoon is in
general of greater strength than that of the cold season, because
being usually a sea wind the air is fully charged with moisture,
condensation takes place as ascensional movement sets in on
reaching the land, and the latent heat set free strengthens the
upward current.

The position, outline and relief of the continent of Asia favour the development of
monsoons to a much greater extent than any other part of the world;
so much so that the climate of the whole of the southern and
eastern parts is entirely controlled by these winds, forming what
is typically known as "the monsoon region," a region having
distinctly characteristic products. Monsoons form an important
element in the climate of Australia, western and southern Africa, and the southern part of
the United
States of America, but
with a few exceptions the monsoons of those regions are local in
character, modifying the prevailing winds of the planetary
circulation (usually the trade winds) for a longer or shorter
period every year.

Mechanism

The mechanism is simple: land heats up faster than water. The temperature difference between the land and sea can be as much as 20°C – land temperatures in India can be hotter than 45°C, while the surrounding water in the Bay of Bengal and the Arabian Sea remains in the low 20s. The heat absorbed by the land warms the air above it. The hot air rises, and cooler ocean air rushes inland to replace it. As it moves, it carries moisture with it, releasing it over land as the summer monsoon (also known as southwest monsoon).

The hot air over the land tends to rise, creating an area of low pressure. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it.[1]rainfall is increased by the moist ocean air being lifted upwards by mountains, as with the Tibetan Plateau,[2] and by surface heating.[3]